This invention relates to a composite material having a controllable mechanical property. More particularly, the invention relates to a composite meta-material with a property that may be altered after fabrication and during usage.
Historically, man was initially limited in materials selection to what was available around him: wood, stones and bones. He eventually gained the ability to refine naturally occurring materials such as iron and bronze and to mold and shape these materials. A few millennia later, man invented custom materials and composites, such as plastics and reinforced steel, whose mechanical properties could be tailored during fabrication for a specific application.
Revolutions in materials technology led to applications revolutions. The Iron and Bronze Ages produced shaped weapons, farm tools, jewelry, and eating utensils. Composite materials at the turn of the 20th-century enabled a wide array of new applications. Flight leveraged new lightweight and high strength materials; steel-reinforced concrete built bigger buildings and bridges; and plastics led to revolutions in toys and other industries.
These materials allowed the designer or builder to select from a wide range of mechanical properties for a given application. However, once the material is selected and incorporated into a device or structure, its mechanical properties are fixed. The ability to actively control a mechanical property of a material during usage would be useful in many applications—and enable many new ones. Existing materials that can vary a mechanical property are still very limited and may be divided into two categories: active materials and intrinsically adaptive materials.
Intrinsically adaptive materials undergo transformations in their molecular or microscopic structure in response to external stimuli, which results in a mechanical property change. Examples of intrinsically adaptive materials include thermally responsive materials, such as rubber and shape memory polymers, where stiffness and damping vary based on temperature; magentorheological and electrorheological fluids where the material undergoes a microstructural transformation in response to an external magnetic or electric field; and polymer gels where the stiffness changes depending on the amount of fluid in the polymeric matrix. These materials can exhibit undesirable temperature sensitivity. Also, these materials provide limited control. For example, it is not possible to independently vary elasticity and damping for these materials or to control an electrorheological fluid between liquid/solid extremes.
Active materials act as energy transducers that convert between electrical (or thermal) energy and mechanical energy of deformation. Examples of active materials include piezoelectric ceramics, magnetostrictive materials (including ferromagnetic shape memory alloys), and electroactive polymers. For these materials, their particular energy conversion mechanism often limits the range of mechanical properties that can be obtained. In addition, control of a mechanical property for an active material is subject to physical limits, such as maximum energy output and speed of response for the active material.
Based on the foregoing, materials selection is still limited and materials with one or more controllable mechanical properties largely remain an unmet need.